Bulk heterojunction (BHJ) polymer solar cells (PSCs) composed of conjugated polymer donors and small or macromolecule acceptors have been of interest in the development of organic photovoltaic cells. The bicontinuous nature of the phases in BHJ creates a large surface to volume ratio for efficient exciton dissociation, and the facile and low-cost fabrication is also appealing. Despite extensive efforts to improve the properties of the constituent materials and morphologies of BHJ systems, the lower power conversion efficiency (PCE) as compared to silicon-based solar cells remains a challenge. Researchers have developed low band gap polymers that absorb a high fraction of the solar spectrum and assemble into desirable film morphologies. Efforts to improve n-type acceptor components have included BHJs with small molecules such as 9,9′-bifluorenylidenes, perylenediimides, and vinazenes. Nevertheless, fullerenes are widely used in part due to their high electron affinities and low reorganization energies in the electron transfer reactions. Reactions with organometallic reagents, radicals, and transition-metal complexation have been investigated to create new fullerenes for BHJs. The reactivity of fullerene resembles that of an electron-deficient polyolefin, and as such, a dominant functionalization approach has been to use cycloaddition reactions to modify the fullerene structure. Noteworthy examples include the synthesis of PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) via 1,3-dipolar cycloaddition and the indene-C60 adducts from Diels-Alder reactions. However, reacting fullerenes often requires an excess of reagents, long reaction times, and high temperatures.
Less-than-optimal band offsets at the BHJs can lead to energy loss and low open-circuit voltages in solar cells. In order to address this issue and increase power conversion efficiencies, there have been efforts to increase the HOMOD-LUMOA (A: acceptor, D: donor) gap which is proportional to the open-circuit voltage. Many polymer donors have been designed for optimized absorption of the solar spectrum by having a repeating structure with alternating electron-rich and electron-poor units. However, lowering the HOMO levels in these systems is often complicated by the fact that simple modifications of the electron-rich repeating units can lead to an expanded band gap that reduces the solar absorption efficiency, thereby giving a smaller short-circuit current density (JSC). Another approach is to raise the LUMO level of the fullerenes, a strategy which offers a more predictive route to increased VOCs. Most functionalization methods, including cycloaddition reactions, break the full conjugation of the C60 π-system and generally decrease the electron affinity and raise the LUMO level. The decreased relative electron affinity of C60 is measured by the change of the onset reduction potential or the calculated LUMO level. For example, indene-C60 has been shown to exhibit ca. 100-200 mV reduced electron affinities (higher LUMO) relative to C60.